In spite of the simplicity of the geometrical argument for the PA traverse, many open questions remain. In about half of all pulsars, the PA traverse does not conform to the RVM and reasons for this must be sought. The main explanation comes from propagation through the magnetosphere, including refraction of the modes [1]}, [2]}, [3]}, coherent mode mixing [4]} and generalised Faraday rotation [5]}, [6]}. In addition, it is likely that the various components of the pulse profile arise at different emission heights, with outer components originating from higher in the magnetosphere [7]}, [8]}, [9]} where sweepback of the magnetic field lines might be important [10]}. Finally, the dipolar field may be offset from the star's centre and/or quadropole or higher-order fields may be important (e.g. [11]}). Furthermore, even for pulsars which show PA traverses compatible with the RVM, the narrow duty cycle of the pulse profile makes it difficult to determine a unique solution for \(\alpha \)  and \(\beta \)  in the absence of further constraints (e.g. [12]}).
